This invention relates in general to gate valves, and in particular to a gate valve having an asymmetrical gate that allows shearing of a wireline while maintaining post-shear seal integrity.
In the prior art, two types of coatings are generally used on valve gates. Some gates are coated with a layer of very hard material such as a carbide material. A coating of very hard material offers great durability. However, use of this type of coating on gates that are used to shear a wireline is not recommended. A coating of very hard material is generally brittle, thereby being inherently subject to chipping. Also, this type of coating is generally thin, averaging between 0.003 inches and 0.005 inches, and incapable of holding an edge while cutting. Furthermore, since this coating is not metallurgically bonded to the substrate material, high shear stresses that arise at the coating-substrate interface promote cracking of the coating. Cracking or chipping of the coating is not desirable because it reduces sealing efficiency, thereby requiring replacement of the gate more frequently.
Since coatings of very hard materials, such as carbides, are not ideal for wireline cutting applications, wireline shearing gates have been typically hardfaced with a second type of coating. The type of coating that is more suitable for wireline cutting operations is a hard ductile material such as Stellite(copyright) or Colmonoy(copyright) to provide protection against chipping when used for shearing. However, sometimes it is difficult to coat larger areas with these materials without cracking of the coating. Also, such ductile materials have markedly inferior wear characteristics compared to carbides and are easily scratched or otherwise damaged.
Because of the above problem with coating or hardfacing gates with either only an extremely hard material or only a more ductile material, prior art gate valves have not been suited for shearing wireline while retaining post-shear seal integrity.
Also, prior art seat seals have used PTFE (Polytetrafluoroethylene) jackets energized with a stand off ring that inserts within an opening in the seat seals. The openings in the seat seals have traditionally faced towards the gate of the gate valve. A problem with this configuration is that while pressure acting on the downstream seal acts to force the seal open, thereby energizing the seal, pressure on the upstream side of the gate acts to compress the seal, which results in leakage around the seal.
In this invention, the single shear gate of a gate valve is coated with a combination of materials to achieve a gate capable of shearing wireline while retaining seal integrity. The asymmetrical gate allows for shearing wireline in a single location, thereby eliminating a slug of shearable material. Since ductility is desired at the shearing edge of the gate, and extreme hardness is desired at the sealing surfaces of the gate, this invention strategically locates materials having appropriate characteristics.
The shearing edge is constructed of an inlay of a hard ductile material that provides protection against chipping. The sealing surfaces, on the other hand, are coated with an extremely hard material that provides durability to the sealing surface. Extremely hard sealing materials are very brittle and may crack and chip if subjected to the high shearing stresses encountered during shearing. However, cracking and chipping is prevented by providing inlays of a more ductile material located at the shearing edges that isolate the brittle sealing material from the majority of the shearing stresses.
Valve seats surrounding the gate of the gate valve have seals provided to seal between the valve seat and the valve body. The seals are energized with a standoff ring. The upstream seal is reversed from the traditional orientation such that the standoff ring engages the valve body and the seal opening faces upstream, which results in pressure energizing the seal, thereby preventing leakage.